1. Field of the Invention
The present invention relates to a polyester fiber which can be dyed dark with disperse dyes in a low temperature, atmospheric pressure dying machine at a temperature of not higher than 100.degree. C. and is superior in dye fastness.
2. Description of the Prior Art
Polyester fibers, of which polyethylene terephthalate is representative, are widely used in clothing because of their excellent physical and chemical properties and thermal resistance. At present, however, polyester fibers dyeing can be performed only at high temperatures under high pressures because of their chemical structure, in which the radicals capable of reacting with dyes are absent; and of their dense physical structure which is too dense to allow disperse dyes to penetrate into the polyester fibers.
Typically, polyester fibers are dyed at as high as 130.degree. C. and thus, are limited in being mixed with thermal resistance-insufficient fibers such as spandex or natural fibers. As a result, polyester fibers are restricted to various uses and products.
Reduction of the dyeable temperature to 100.degree. C. or less enables polyester fibers to be used in mixture with spandex or natural fibers, to be dyed with dyes of low thermal resistance, such as reactive dyes, and thus to be mixed with various materials.
As for nylon fibers, they, even if dyeable at 100.degree. C. or less, suffer from the problem of being apt to undergo yellowing by heat and UV light and even by washing.
In contrast, polyester fibers do not show yellowing, but require high temperatures and pressures and thus high costs for their dyeing. To overcome this problem, extensive attempts have been made to develop easy dyeable polyester fibers which can be dyed by use of a low temperature, atmospheric pressure dyeing machine. For example, the development of easy dyeable polyester fibers has been approached through high speed spinning techniques or copolymerization techniques. One of the high speed spinning techniques is disclosed in U.S. Pat. No. 4,134,882 in which polyester is spun at high speeds by use of a spinneret with capillary-dimensions that produce high shear as the molten polyester resin is extruded. Thus, an alternation is generated in the orientation and crystalline structure of their molecular chains, leading to a change in the morphology of the polyester fibers and thus an improvement in their dyeability. The polyester fibers prepared according to the reference patent are able to be dyed at low temperatures, but are problematic in that an expression of a deep color on them requires the use of a high pressure dyeing machine at temperatures as high as 100-120.degree. C.
On the other hand, copolymerization techniques for improving the dyeability of polyester fibers have the advantages of being conducted without requiring new production facilities, and not being limited to polyester fiber processing. Available for the copolymerization techniques are lipid compounds such as hydrocarbons, aromatic hydrocarbons and polyesters, which have two functional groups, such as two hydroxy groups or two carboxylic groups (ester groups), through which a chain is introduced into the backbone of the polyester. Of such compounds, those which have hydrophilic functional groups can be grafted with the greatest ease. In this connection, some techniques are found in Japanese Pat. Appl'n No. Sho. 61-226510 (Japanese Pat. No. 63085111) and Japanese Pat. Laid-Open Publication No. Hei. 4-41730. The polyester fibers manufactured according to these reference patents can be dyed at atmospheric pressure, but suffer from disadvantages of restricting the available dyes because of their poor light fastness as compared with common polyester fibers; and being apt to be not dyed uniformly, but locally.
Recently, extensive research has been performed on polytrimethylene terephthalate fibers. Polytrimethylene terephthalate fibers are characterized in that their initial elastic modulus is by about 30 g/d lower than that of nylon 6 fibers; are dyeable at lower temperatures than are polyethylene terephthalate fibers; and are of excellent elastic recovery. Dyeable as it is, polytrimethylene terephthalate requires temperatures as high as or higher than 105.degree. C. for the expression of dark colors thereon. Also, it is not reasonable to regard the dyeing of polytrimethylene terephthalate as atmospheric pressure dyeing.
In Japanese Pat. Laid-Open Publication No. Hei. 11-100722, there is an easy dyeable polyester fiber which is deeply dyeable at less than 100.degree. C. with disperse dyes, and superior in color fastness, and allows spandex to be weaved in mixture without yellowing the spandex. However, since the polyester fiber consists of at least 90 mol % of polytrimethylene terephthalate units and 0-10 mol % of ester units, use of such a high content of expensive polytrimethylene terephthalate is economically unfavorable. In addition, the presence of as much as 90 mol % of polytrimethylene terephthalate decreases the melting point of the copolymer and forms spots in the dyeing on the filament upon after-treatment (e.g., dyeing).